Astronomers find fifth moon at Pluto

The Hubble Space Telescope has revealed a new moon, called P5, orbiting… (NASA, ESA and Mark Showalter…)

Astronomers using the Hubble Space Telescope have discovered a fifth moon orbiting the icy dwarf planet Pluto, a surprise because they did not expect such a complex system surrounding such a small body.

The new moon, designated P5, is thought to be irregular in shape and six to 15 miles across. It is in a 59,000-mile-diameter circular orbit around Pluto, which was demoted from its status as a full-fledged planet in 2006 following the discovery of other similar-sized bodies in the Kuiper belt. The belt is a region of space on the outer fringes of the solar system that contains many small icy objects and a number of dwarf planets.

The five moons orbiting Pluto "form a series of neatly nested orbits, a bit like Russian dolls," said astronomer Mark Showalter of the SETI Institute in Mountain View, Calif., who led the team that discovered P5. The moons are thought to be relics of a collision between Pluto and some other large object in the Kuiper belt billions of years ago.

Pluto's largest moon, Charon, was discovered in 1976. Hubble observations in 2006 revealed two other moons, Nix and Hydra, and more observations in 2011 revealed the fourth moon, known provisionally as P4.

New Horizons, a NASA space probe, is on its way to Pluto and is scheduled to make a high-speed flyby in 2015. It will return the first detailed images of the system, whose components are so small and distant that even Hubble can barely see the largest features on the dwarf planet's surface.